txstatsd: gather runtime self metrics under statsd

this gets rid of top-level txstatsd metrics and put them under common statsd/ (at the moment swift machines are pushing their stats to top-level)

once this is rolled out we can backfill previous data with whisper-fill
from https://github.com/jssjr/carbonate

Change-Id: Ie05a49bcfb7e9fbc1e2b493bbfe301e84dc35fe2